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TO-220 · LM317
ActiveAdjustable Linear Regulator
LM317
Texas Instruments · TO-220 · Adjustable Linear Regulator
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Overview
The LM317 is the adjustable companion to the fixed 78xx family and still the default answer whenever a design needs an odd rail: 1.25 V to 37 V at more than 1.5 A, set by a two-resistor divider from a 1.25 V bandgap reference that is guaranteed to within 1.2 V to 1.3 V across line, load and temperature. TI's current datasheet (SLVS044Z) rates the input-to-output differential from 3 V to 40 V, quotes 0.01%/V typical line regulation and 0.1% typical load regulation, and keeps adjustment-pin current at 50 µA typical and 100 µA maximum — small enough that the standard VOUT = 1.25 V x (1 + R2/R1) + IADJ x R2 equation stays accurate with 1% metal film resistors.
Because the device only ever senses the differential between input and output, it is a floating regulator: you can stack it on a negative rail, use it as a current source by fixing R1 and putting the load in series, or clamp the adjustment pin to ground for a 1.25 V output with electronic shutdown. Protection is generous for a three-terminal linear part — current limit sits between 1.5 A and 2.2 A for differentials up to 15 V and folds back to about 400 mA at 40 V, thermal overload shuts the pass device down at 150 °C junction, and output safe-area compensation keeps the transistor inside its SOA during startup and shorts. Minimum load current is 3.5 mA typical and 10 mA maximum, so a 240 Ω R1 from output to adjustment pin (about 5 mA) is the conventional choice and should never be removed.
Thermal design drives every LM317 decision. TO-220 junction-to-ambient is 23.5 °C/W on the KCS variant and 37.9 °C/W on KCT, junction-to-case through the tab is far lower, and the junction limit is 150 °C. A 24 V to 12 V conversion at 1 A is 12 W, which is impossible without a large heatsink; the same rail from an LM2596 buck wastes under 1.5 W. Ripple rejection is 62 dB to 64 dB at 120 Hz and rises to about 80 dB with a 10 µF capacitor on the adjustment pin, which is the trick that makes the LM317 usable as a low-noise supply for analogue front ends and VCOs.
Sourcing: TI, onsemi, STMicroelectronics (LM317T), Diodes Incorporated and Asian second sources all ship the same pinout — adjustment, output, input on TO-220 with the tab as output. TI also offers TO-263 (KTT) and 4-pin SOT-223 (DCY) versions at 800 mA-class dissipation, and the LM317L/TLV variants for low current. MOQ from our Shenzhen stock is 50 pcs loose or one tube (50 pcs) for TO-220, shipping in one to two business days; authorised TI channel replenishment is eight to sixteen weeks. Counterfeit risk is moderate: verify laser etching, check that adjustment-pin current really is near 50 µA (clones often draw 300 µA or more, wrecking accuracy at high R2 values), and load-test at 1.5 A.
Key Features
- 1.25 V to 37 V adjustable output at more than 1.5 A with two external resistors
- 1.25 V reference held to 1.2 V - 1.3 V over line, load and temperature
- 0.01%/V typical line regulation and 0.1% typical load regulation
- Current limit, thermal overload and safe-area compensation built in
- Floating architecture also works as a precision current source or negative-rail regulator
Typical Applications
- Bench and lab power supplies with a potentiometer-set output
- Odd rails (6 V, 9 V, 13.8 V radio, 15 V analogue) from a 24 V or 28 V bus
- Constant-current drivers for LED strings, battery trickle charging and relay coils
- Low-noise analogue and RF rails with a bypassed adjustment pin
- Motor and actuator test rigs, automotive accessory supplies
Specifications
- Output Voltage Range (Adjustable)
- 1.25 to 37 V
- Input-to-Output Differential Voltage
- 3 to 40 V
- Output Current (Max)
- 1.5 A
- Reference Voltage (Typ)
- 1.25 V
- Line Regulation (Typ)
- 0.01 %/V
- Load Regulation (Typ)
- 0.1 %
- Adjustment Pin Current (Max)
- 100 µA
- Current Limit (VIN - VOUT ≤ 15 V)
- 1.5 to 2.2 A
- Pins
- 3
- RoHS
- Compliant
Procurement Tips
Always pair the LM317 with a specified heatsink and a 240 Ω R1 on the same BOM line, and quote a switching alternative such as the LM2596 or MP1584EN when the input-to-output differential exceeds 5 V at more than 500 mA — the linear solution will not survive an enclosed cabinet at 60 °C ambient.
Frequently Asked Questions
LM317 versus LM7805 — when do I use which?
Use the LM7805 when the rail is fixed at 5 V and you want the lowest part count; use the LM317 when the output must be adjustable or is anything other than a standard 78xx voltage. The LM317 has better regulation (0.01%/V line versus about 5 mV to 100 mV for a 7805), better ripple rejection with a bypassed adjustment pin, and it needs a minimum load of 3.5 mA to 10 mA that the fixed part does not. Both drop about 2 V to 3 V, so neither is efficient from a high input — that is the LM2596's job.
How do I calculate R1 and R2 for a target voltage?
VOUT = 1.25 V x (1 + R2/R1) + IADJ x R2. Fix R1 at 240 Ω, which draws about 5 mA and satisfies the 3.5 mA to 10 mA minimum load over temperature, then R2 = R1 x (VOUT/1.25 - 1). For 12 V that is 240 x (9.6 - 1) = 2.06 kΩ, so use 2.05 kΩ 1% or 2 kΩ plus a 100 Ω trim. Keep the IADJ x R2 term in mind when R2 exceeds about 5 kΩ: 100 µA through 5 kΩ is another 0.5 V of error, so use a lower divider impedance or an op-amp buffer.
Why does my LM317 output sag or oscillate under load?
Three usual causes. First, insufficient input headroom — the part needs 3 V of differential, and a transformer-derived rail that sags below VOUT + 3 V will drop out. Second, no minimum load: with R1 open or a very high divider impedance the reference current starves and the output drifts up or collapses. Third, poor layout — the 100 nF input capacitor must sit close to the input pin and the output capacitor close to the output pin, with the adjustment-pin divider returning straight to the output capacitor ground, not to a noisy load return.
Can I use the LM317 as a constant-current source?
Yes, and it is one of its best roles. Put the resistor between output and adjustment pin (R = 1.25 V / I, so 1.25 Ω for 1 A) and the load between adjustment pin and ground; the device holds 1.25 V across the resistor regardless of load voltage up to the differential limit. Add the 50 µA to 100 µA adjustment-pin current to your budget when accuracy matters, and remember the resistor dissipates 1.25 W at 1 A, so use a 2 W part. For higher currents an LM350 (3 A) or a switching LED driver is more efficient.
What is the difference between LM317, LM317L, LM317M and LM317T?
Same circuit, different packages and current classes. LM317T is the TO-220 version rated for 1.5 A with a heatsink, LM317M/LM317S are the TO-263/D2PAK surface-mount versions, LM317L is the TO-92 or SOIC small-signal version rated around 100 mA, and the DCY SOT-223 option handles 800 mA. TI also distinguishes a legacy chip from a new chip with slightly different PSRR (57 dB versus 80 dB with a 10 µF adjustment capacitor) — check which one your order code maps to before relying on ripple-rejection numbers in a precision design.
How do I avoid counterfeit or remarked LM317 parts?
Buy from authorised channels or from a distributor that electrically tests incoming lots, which we do. On inspection, look for laser-etched TI or ST logos with consistent font and a traceable date code, matte tin leads and clean moulding. Electrically, measure adjustment-pin current (50 µA typical, 100 µA maximum) and load the part to 1.5 A: remarked LM317L or LM1117 die cannot carry the current and will fold back early or overheat quickly. Ask for the certificate of conformance and reel or tube photos before dispatch.
What is the MOQ and lead time?
MOQ is 50 pcs loose or one tube (50 pcs) for LM317T in TO-220; tape-and-reel quantities start at 800 pcs for TO-263 versions. Shenzhen stock ships in one to two business days, while authorised TI replenishment is typically eight to sixteen weeks.
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